40 



LABORATORY MOUSE 



If the double heterozygotes be mated back to the parent 

 pure for the two recessive characteristics, four classes of 

 offspring will be expected in equal numbers, namely, normal- 

 eyed pigmented, normal-eyed albino, rodless pigmented, rod- 

 less albino. This test (87) yielded of those classes respectively 

 66: 54: 68: 51 or ratios of 1.1: .99: 1.1: .93. 



Gametes of double heterozygotes 

 RC Re rC 



Gametes of re 

 double 



Found 



Expected 



The gene for two or more unit characters may lie at differ- 

 ent points or "loci" upon the same chromosome, a relation- 

 ship known as linkage. Linkage is measured by the amount 

 of crossing-over, or recombination. Ordinarily in the forma- 

 tion of gametes the two chromosomes of an homologous 

 pair, which have lain side by side in immature germ cells, 

 separate at maturation and one passes to each daughter cell. 

 Each functional gamete (egg or sperm) thus contains one 

 representative (not two) of each gene. The chances are even 

 that it will be the representative derived from the father or 

 that derived from the mother. 



Occasionally chromosomes break (see Fig. 7) transversely 

 while lying elongated in pairs previous to the reduction 

 division. When the break is repaired the broken ends may 

 become united differently from before. The chromosomes 

 which originally bore RC and re respectively may after a 



